The field of the invention is medical devices. More particularly, the invention relates to medical devices applicable to radiosurgery.
Radiosurgery is a non-invasive medical procedure that ablates a targeted tissue by ionization of targeted cells with high-energy beams of radiation. The ionization process causes ions and free radicals to be released within the target cells and ultimately results in cell death. As such, it is important that the irradiation is directed only to targeted cells, such as tumors or other targeted cell masses. Commonly, however, healthy tissue surrounding the target cells are damaged in the process due to various difficulties of existing technology that are identified as follows.
Linac-based stereotactic radiosurgery involves ejecting a beam of electrons from an electron gun and delivering an acceleration energy to the electrons in the form of microwaves. The electrons subsequently impact a metal target, producing x-rays. In some linac-based systems, the highly energized electron beam is redirected by application of magnetic fields before striking the target, and the resulting x-rays travel toward a modification center where the beam is treated prior to delivery to the target cells. A common modification process involves colliding the beam by placement of a collimator interrupting the beam pathway such that an opening in the collimator allows for passage of only radiation that is directed towards the intended target. A secondary collimator placed downstream provides further refinement in shaping the beam by absorbing x-rays as required. Radionuclide based radiosurgery is similar, with the exception that instead of electrons striking a metal target to produce x-rays, naturally occurring x-rays from the radionuclide, typically 60Co, act as the radiation source. Beam modification as described above then proceeds in an analogous manner as with linear accelerator based radiosurgery.
Generally, two classes of collimators are available for application in the linac-based system, including circular cone collimators and multileaf collimators. Typically, the circular cones are used for lesions in the 4 mm to 30 mm range, while the multileaf collimators are preferred for larger or more complex lesions that would require a complex radiation beam shape that is achievable by dynamic positioning of the metal leaflets during treatment. The multileaf collimator, however, is limited to a more shallow penumbra and more gradual dose falloff around the target. On the other hand, the circular cones are able to achieve a steeper dose falloff and thereby spares more of the surrounding healthy tissue. What is needed in the standard conical collimator is a means for increasing both the dose gradient and dose uniformity in the radiosurgery process.
In stereotactic radiosurgery of lesions in a brain, precisely directed radiation is very important. With the so-called Gamma Knife system, several radioactive sources surround a patient's skull with a specialized collimator placed between the sources and the skull. The radiation beams that pass through the collimator converge at a predetermined point inside the patient. For instance, a radioactive cobalt source may emit a plurality of gamma rays directed toward a helmet surrounding the patient's skull, whereby the rays are collided by the helmet such that only certain rays having a common delivery point are delivered to the patient's skull. Stereotactic radiosurgery systems typically utilize circular cone collimators for modifying the emitted rays.
Since any damage to healthy brain tissue may have undesired health implications, the irradiation is highly selective and the dose gradient beyond the edges of the radiation field is a key concern during treatment planning phases of stereotactic radiosurgery. Typically, the volume of brain receiving a certain dose, such as 12 Gy, is monitored as a plan quality metric since such dosimetric parameters have been correlated to toxicities. Alternatively, a dose gradient index can be determined as the ratio of the volume receiving 50% of the prescription dose to the volume receiving the full prescription dose. This plan quality metric has been correlated to toxicities, for example in radiosurgery for meningioma. Additionally, the homogeneity of the dose distribution inside the target, as measured by the maximal point dose, or as an integrated dose to a clinically relevant sub-volume of the target, has been correlated to toxicities after treatment for benign diseases such as vestibular schwanoma and meningioma with stereotactic radiosurgery. Given the above two examples of clinical correlates in the plan quality metrics, what is needed is a technological development that provides an increase in dose gradient and/or an increase in dose homogeneity for improving patient care in stereotactic radiosurgery.
In general, there are two main considerations that influence the penumbra and uniformity in the dose distribution for conical collimator based radiosurgery. First, the radiation source is not truly a point source and therefore is amenable to blurring in the dose distribution. Blurring may be minimized by placement of the collimator as close as possible to the target volume. Second, the transport of secondary electrons and scattered photons away from a primary interaction point leads to additional blurring in the dose distribution. Both of these two physical phenomena can be incorporated into a pencil beam dose calculation formalism. Given that these phenomena are relatively well characterized in regard to prior knowledge of the shape of the effective dose kernel, a fluence pattern could theoretically be designed to optimize both dose gradient and uniformity. Sharpe et al. (M. B. Sharpe, B. M. Miller and J. W. Wong, “Compensation of x-ray beam penumbra in conformal radiotherapy,” Med Phys 27, 1739-1745 (2000).) have developed a technique for increasing dose gradient for larger fields using modulation by introducing a larger fluence at beam edges. The technique of Sharpe et al., however, considered larger fields that were sized for use in conventional lung radiotherapy, and further, the optimal size and intensity of the additional fluence at beam edge was arrived at empirically. What is needed is a method addressing smaller fields having an optimized fluence distribution.
Therefore, it would be desirable to have a system and method to address the above concerns and to provide related advantages.